Multiepoch infrared interferometric observations of evolved stars at the VLTI

This thesis presents multi-epoch interferometric observations of evolved stars on the Asymptotic Giant Branch (AGB). The Asymptotic Giant Branch is populated by low to intermediate mass stars (0. 6 – 10 Mo) in the final stage of their stellar evolution. Pulsating AGB stars suffer from strong mass loss via a dense and dusty outflow from an extended stellar atmosphere returning gas and dust to the interstellar medium. The processes leading to the violent mass loss are not well understood and are therefore under intense investigations. Due to the AGB stars’ large diameters and high luminosities, evolved stars are ideal targets for high angular resolution observations. The AGB stars presented in this thesis were observed over several pulsation cycles using the near-infrared AMBER and mid-infrared MIDI instruments at the Very Large Telescope Interferometer (VLTI). The goal of this study is to investigate the connection between the pulsation mechanism, the dust chemistry of AGB stars, and the condensation sequence in order to better understand the mass loss process. The thesis studies a sample of four evolved stars, the oxygen-rich Mira variables RR Aql, S Ori, GX Mon, and R Cnc. We obtained multi-epoch MIDI observations and individual AMBER observations. The presented data are modelled using basic models of uniform disks and Gaussian profiles. The AMBER data, along with the basic models, are modelled using dust-free self-excited dynamic model atmospheres (P and M series). These modeles represent the stellar atrmosphere including the continuum forming atmosphere and overlying molecular layers (Ireland et al. , 2004b,a). In addition to the basic models, the MIDI data are fitted by a radioactive transfer model of the circumstellar dust shell where the central stellar intensity profile is described by the previously mentioned series of dust-free dynamic model atmospheres, which are based on self-excited pulsation model. The radiative transfer model is computed using the radiative transfer code mcsim_mpi (Ohnaka et al. , 2006). The modelling approach follows the work presented by Wittkowski et al. (2007). For all stars, two dust species of silicate and Al₂O₃ grains were examined. In order to investigate the expected variability of our mid-infrared photometric and interferometric data, model simulations using variations of model phase and dust shell parameters were performed. The observed visibility spectra for all studied stars do not show indications of variations as a function of pulsation phase and cycle within our investigated phases and uncertainties (5%-20%). The observed photometry spectra may indicate intra-cycle and cycle-to-cycle variations with a significance of 1-5σ. However, the photometry variations cannot be confirmed within our uncertainties. Our study shows that the photometric and visibility spectra for all stars can be well described by the radiative transfer model of the dust shell that uses a dynamic model atmosphere describing the central source. For all epochs, we found the best fitting models consisting of a dynamic model atmosphere, and dust shells parameters including the optical depth of the dust shell, the inner radii, the power-law index of the density distribution, and the photospheric aangular diameter. The optically thin dust shell did not improve the model fit. However, the model simulations indicate that the presence of an inner Al₂O₃ dust shell with low optical depth compared to the silicate dust shell can not be excluded. GX Mon can be modelled with a combination of Al₂O₃ and silicate grains. The dust shell of S Ori and R Cnc can be well modelled using only Al₂O₃ dust grains without a contribution of silicates. The inner boundary radii of Al₂O₃ appears around 2-2. 5 photospheric radii and the inner moundary radii of silicate appears around 4-5 photospheric radii. The modelling simulations performed in this thesis confirmed that significant intra-cycle and cycle-to-cycle visibility variations are not expected at mid-infrared flux and visibility values are expected to be less that 25% and 20% respectively, and are too low to be detected within the measurement uncertainties. The study represents the first comparison between interferometric observation and a combination of a radiative transfer modelling with dynamic atmosphere models over and extended range of pulsation phases covering several cycles.